The 3BZ is the name of a transmitter/receiver
radio set in the TELERADIO series designed and manufactured by A.W.A. (Amalgamated
Wireless Australasia) over several years. The 3B and 3BZ types were used extensively
in the 1940s by "Coastwatchers" who lived in the jungle on Pacific islands,
behind enemy lines, and reported shipping and aircraft movements. They were
also used by Coastal Radio Stations, fire brigades, and other civil services.
In the 3BZ radio station, the transmitter was the J50062 (labelled 3BZ) and the receiver used was the C6770 (without the 3BZ label).
There were several receiver versions, which included circuit changes, crystal locking, different case styles and different types of power supply. There is also a C6940 General Purpose Communications Receiver that looks very similar but was not part of the 3BZ Teleradio.
The mark on the top is from a piece of old masking tape which had been there for many years.
PERFORMANCE
The radio is fairly sensitive on all
but the highest range, not bad for a pre-war design. It is not as sensitive
as a communications receiver, as it only has one RF amplifier. This example
provides a sensitivity on its Ranges of:
A 6 microvolts,
B 4 microvolts,
C 2-3 microvolts,
D 2-5 microvolts,
E 4-10 microvolts,
(for a signal to noise ratio of 10 dB)
It makes a good short wave listener's radio,
and has very simple controls. It is easy to tune, with the slow motion dial,
but I find the dial pointer and no illumination not the easiest to use. Vibrator
hash is low. It can resolve SSB signals even with the fixed BFO. The RF gain
needs to be varied to adjust the BFO injection for different strength SSB
signals. This causes the BFO to change frequency which is annoying, but after
a while you develop a feel for it, and can use this interaction to tune the
BFO frequency.
This would perhaps be the ugliest radio I have ever seen! If it wasn't for its use with the Coastwatchers to place it in the history books, it may have vanished without a trace. It must be one of the ruggedest radios built and would probably survive any drop test, with perhaps just the valves falling out. The valves are retained only by their socket pins and the valve shields.
CONTROLS
The front panel is dominated by a large
semi circular dial scale and a Muirhead slow motion drive and tuning knob.
The outside knob ring gives a 1:1 drive and the centre knob gives a 56:1
reduction ratio. The knob cover and dial pointer are easily broken. The frequency
scale is etched on the semicircular brass plate with the frequency coverage
and the letter of the range. All the other controls are located in a horizontal
line at the bottom of the front panel. There is an etched strip that carries
the knob labels and model and serial numbers. Some models also had a pilot
light and a name tag in the top left and right hand corners. The left hand
knob is the audio gain control labelled AF VOL., next is the TONE control,
and then the 3 position function switch that is labelled OFF, STANDBY, RECEIVE.
In STANDBY the valve heaters are on and the HT is off. To the right of the
centre tuning knob is the mode control which is labelled PHONE, MCW, CW.
In the PHONE position, the AVC is on. In the the CW position the BFO
is turned on. There is no BFO frequency adjust control. The next control
is the frequency selector switch that is labelled RANGE and has 5 positions
marked A, B, C, D, E. The ranges are:
A 30 to 9 Mcs
(10 to 33 meters),
B 11.1 to 3.5 Mcs (27
to 85 meters),
C 4.6 to 1.5 Mcs (65
to 200 meters),
D 1650 to 545 kcs (185
to 550 meters),
E 515 to 200 kcs (580
to 1500 meters).
The last control at the extreme right
is the RF gain control which is marked RF VOL.
The speaker, headphones, aerial and earth, are connected by passing wires through holes in the sides and back of the steel case and connecting them to press terminals. When the lid is removed it is easy to feed the wires through and press the terminals down for connection. They are all clearly labelled internally. The holes have a small rubber flap to prevent dust entry. The power is attached at the LHS lower back. This example is a 6 volt DC model and has a male chassis mounted 8 pin octal plug. The circuit diagrams show a 4 pin plug.
DIFFERENT MODELS
The receiver came in several versions
denoted by a number in front of the C6770 model number. They are:
C6770 Battery operated,
6 or 12 volt, 30 Mcs to 200 kcs.
1-C6770 Battery operated,
6 or 12 volt DC, 30 Mcs to 200 kcs, in R6836 case.
1C6770 A.C. mains
operated, 105-130 or 200-260 volt, 30 Mcs to 200 kcs.
2-1C6770 A.C. mains
operated, 105-130 or 200-260 volt, 30 Mcs to 200 kcs, in 1R6836 case.
3C6770 Battery operated,
6 or 12 volt, 11.1 Mcs to 200 kcs, and a crystal locked position.
3-3C6770 Battery operated,
6 or 12 volt, 11.1 Mcs to 200 kcs, and a crystal locked position, in 2R6836
case.
10C6770 Battery operated,
6 or 12 volt DC, 30 Mcs to 200 kcs, in 14R6836 rack mount case.
11C6770 A.C. mains
operated, 105-130 or 200-260 volt, 30 Mcs to 200 kcs, in 15R6836 rack mount
case.
The manual also mentions a DC supply mains
version with 110 to 130 volts or 200 to 260 volts but it provides no
model numbers or circuits.
DESIGN
The C6770 is a conventional single conversion superheterodyne receiver using octal valves. It has a 6U7G as the RF stage, a 6J8G as the mixer, a 6J8G as the IF amplifier and BFO, a 6G8G as the detector, AVC rectifier and audio amplifier, and a 6V6GT as the audio power amplifier. The IF frequency is 535 kcs. The receiver with a mains power supply uses a 5Y3GT as the rectifier. The 6 volt DC version uses a synchronous vibrator. On some models a crystal could be used as the local oscillator for Range A to lock the receiver to a single frequency. The IF amplifier is a little unconventional in that if uses a valve normally associated with the mixer function. The hexode section performs the IF amplification, and the triode section is the BFO. The IF has such a high gain that it needs to be neutralised.
There were 3 power supplies available which were fitted internally, a 6 volt DC vibrator supply for portable use, a 240 volt AC mains supply for fixed use and a 110 volt DC supply for shipboard use. There is ample room inside for mounting the mains power supply, which consists of a power transformer and choke and the main chassis is punched for the rectifier valve socket. Alternatively, a small box containing a type H6499 vibrator power supply could be fitted.
The case is a unique design, made of heavy steel, plated, and then painted grey crackle. The case consists of a spot welded main front, sides and back, with removable top and bottom covers. The top cover clips on, and the bottom cover is held on by screws. The case has rounded corners and is nearly indestructible! It is very strong. There is also a front cover that clips on to protect the front panel controls. The front covers are not commonly found. There is a normal style "U" shaped steel chassis screwed inside the steel case. The same style case was used for other radios and test instruments, and came in various sizes. Some cases have ventilation slots covered by mesh, and some have none. There are four rubber feet on the case bottom for normal use. These have usually become hard and flattened, or lost. It was also possible to attach a bottom cover to a top cover with long screws and so a transmitter and receiver could be permanently mounted to each other or fixed to a table.
Mechanically, the radio is very rugged. Electronically, it is very simple. It was designed to be a field portable, reliable radio. Murray, in his book "Hunted, a Coastwatchers Story" mentions that 14 natives were needed to carry the complete 3BZ radio set through the jungle.
The chassis is painted green underneath, and is painted grey on the top. The capacitors and resistors are mounted on two tag boards located below the AF amplifiers and near the BFO. The resistors are the body/tip/dot type and the capacitors are the black pitch encased AWA type with the brown paper label. It has point to point wiring and it quite busy underneath. The RF section has a long rotary gang switch with integral metal shields and coils. Individual bypass capacitors are mounted on the valve pins and are quite hard to access. The RF coils that are mounted horizontally on the shields are difficult to adjust.
RESTORATION
The receiver was in good condition, with only a few parts missing, and few modifications. It was cleaned and the minor modifications reversed. The valves were all good and most components were good, so they were not replaced. The paintwork was rough and scratched, with rust showing through in some places. I decided not to repaint it.
The radio came without a vibrator and it was an unusual type. Not only was it synchronous, but it had a split reed. This vibrator type would normally be used for a radio that required a separate connection to both the low voltage reed on the transformer primary, and the high voltage reed on the transformer secondary. However, in this design, they were both connected to ground. I eventually found a "new" one and when I plugged it in, it buzzed away but produced little HT output. When I opened the vibrator can, I found that the rubber support and turned into a gooey mess after 60 years. I cleaned all this away and made a new one out of foam rubber. The vibrator contacts were nonconducting, even though they appeared to be correctly adjusted and touching. They must have been coated by a film of something from when the rubber disintegrated. I cleaned them with a car type points contact file, until I had continuity again. I replaced the capacitors in the supply. The main HT electrolytic was open circuit, and so was one of the 2 noise suppressers. The supply powered up and gave out the correct voltage. After about a minute I could hear a noise and smell something. When I checked, one of the new 1 kv suppresser capacitors was bulging and very hot. I robbed two capacitors from the EHT circuits of a dead computer monitor screen. These were 1.5 kv and 2.5 kv capacitors and these did not fail. The supply powered up again and the wave forms looked good, until after another minute the vibrator note changed, and the input current went from 1 amp to about 8 amps! There was a short somewhere, not in the power transformer I hoped. After disconnecting almost everything, I opened the vibrator again, and noticed a funny smell. The fibre base of the vibrator had arced across between 2 pins and was glowing red and burning. It was a very low resistance. I ground all the carbon away with a small rotary hand burr, and everything ran properly. I filled the hole with some silicon sealant and assembled the vibrator and power supply, and installed it back in the radio.
I began the alignment with tuning the IF to 535 kcs. It was quite a long way off frequency, and I wondered if someone has deliberately set it low. The IF transformers aligned OK so then I set the BFO frequency and began on the RF ranges. The ranges E, D, and C, aligned properly with the normal capacitive trimmer for the high end and slug trimmer for the low end of the range. The trimmer capacitors used on ranges D and C, are Philips beehive type rotary trimmers. The other trimmers are large piston types, bolted to the chassis. You need a tube spanner to loosen the lock nut, and then the pistons can be pulled up and down easily. I fabricated a tube spanner from a short length of copper water pipe and filed the flats on the inside of one end. Don't loosen the nuts off too much, as when I was part way through the alignment, I turned the radio over, and the pistons fell out! Also check that the trimmers are bolted to the chassis securely, that you havn't loosened them, otherwise the alignment will be inaccurate and the receiver noisy. Ranges B and A have no slug trimmers, so you can only set the top end of the frequency range and hope the bottom end is accurate. Range A proved troublesome as during alignment at the high end, the radio began motor-boating (mixed high and low frequency instability). This was a long chase, realigning, replacing bypass capacitors and valves, and replacing filter electrolytics. It wasn't until I re-read the manual that I found the cause. My manual is undated but has a circuit dated 14 April 1940, and two amendment pages, dated 10th March 1943 and 3rd December 1948. One amendment covers the different case styles, and the other covers neutralisation of the IF amplifier. I had a close look and there was no neutralising capacitor, even though it was shown on the circuit. I added the capacitor and the radio became well behaved. Perhaps this is why the IF stage was miss aligned. I re-did the total alignment as I had disturbed things while looking for the instability. Range A has special instructions in the manual to ensure that you select the correct oscillator frequency and do not use the image. If you choose the wrong one, it will work, but the frequency scale will be off, and the sensitivity poor at the lower end of the range.
There is an 8 pin octal male plug mounted on the rear of the radio. The power is supplied through a cable mounted female octal socket. The circuit shows a 4 pin plug so that two 6 volt lead acid batteries can be used, either connected in parallel or in series. In series, one battery supplies the valve heaters and the other supplies the vibrator to produce the HT. In parallel the heaters and vibrator are connected together. When used with the 12 volt 3BZ transmitter, series operation would be normal. The heater and vibrator current drains are different, so a modern 12 volt battery cannot be used, unless it has a 6 volt tap. The 8 pin plug brings out several connections to allow external HT to be supplied as a further option. For 6 volt operation, jumper the HT pins together to use the internal supply.
I found an old pair of Australian made S.T.C. headphones, fitted a new cord to them, and then placed the receiver on the shelf ready for use.
REFERENCES
CIRCUIT C6770
CIRCUIT 1C6770
CIRCUIT 3C6770
CIRCUIT H6499
LAYOUT C6770, 3C6770
LAYOUT 1C6770
ALIGNMENT FREQUENCIES
INSTRUCTION BOOK NO. 6770R SUPERHETERODYNE RECEIVER C, 1C, 3C6770, Amalgamated Wireless (Australasia) Ltd, 47 York St. SYDNEY
The 3BZ Coast Watchers Wireless Set, Colin MacKinnon
The 3BZ Coast Watchers Wireless Set, Colin MacKinnon, RADIO WAVES, January 1996, Page 3-7.
An Economical Commercial Receiver of Wide Application, Stacy J.B. and Holloway F.S., AWA Technical Review Vol. 5 No. 2, 1940, P 61-75.
Hunted. A Coastwatcher's Story; MURRAY,
M. Adelaide: Rigby, 1967
Written by Ray Robinson VK2ILV
